1881.] NATURAL SCIENCES OF PHILADELPHIA. 285 



Interradials three, elongate ; the first generallj^ the largest 

 phite of the calyx, much narrower than wide, frequently extend- 

 ing in length to the secondary radials, Tlie two plates in the 

 second series are often as long as the first, but rarely' of more than 

 half the width, and hence exceedingly narrow; they rest mainly 

 on the outer edges of the secondary'- radials, and curving upward 

 rise to the level of the top of the arm bases. 



First anal plate higher than the first radials, sustaining upon its 

 upper truncate edge a large plate, and on its lateral oblique sides 

 l^artially supporting two plates which extend upwards, adjoining 

 the secondary radials ; these in turn are followed by a large number 

 of small plates, which are more properly ranked as vault pieces. 



Dome high, more or less p3^ramidal, somewhat inflated toward 

 the posterior side. Apical dome plates very large and prominent, 

 and with rare exceptions tuberculiform. The central piece is 

 the largest plate of the entire body; the six proximal plates 

 somewhat smaller, frequently separated from the central piece 

 by a circlet of small irregular plates, and in old specimens and 

 ver}' large species, occasionallj^ isolated laterally. First radial 

 dome plates very large, the two of the second order small. In 

 raj's with three arms, one platp of the latter is large and suc- 

 ceeded by two small ones in a third range. Posterior side of the 

 dome composed of small plates, protruding in a rounded ridge with 

 a depression on either side ; anal aperture at the upper part of 

 the vault, directed laterall}^ 



Column round, consisting of joints with rounded margins and 

 intermediate thinner joints. 



Geological Position, etc — Agaricocrinus is a strictly Subcar- 

 boniferous genus, and is only found in America. It first appears 

 in the Waverlj^ group, attains its maximum in the Upper Bur- 

 lington, and becomes extinct at the end of the Keokuk period, 

 where it attains enormous dimensions. 



We arrange under it the following species : — 



1855. Agaricocrinus americanus Roemer (Amphoracr. americanus). Lethaea. 

 Geogn. (Ausg. 3), p. 250, PL 4^, figs. 15, a, b; Shumard, 1866, Agaricocr. 

 americanus, Cat. Pal. Foss. N. Amer., pt. i, p. 351; Wachsm. and Spr., 

 1878, Proc. Acad. Nat. Sci. Phila., p. 239. 

 This species is very variable, and has a great vertical range, as well as a wide 

 geographical distribution, being found from the upper portion of the Upper 

 Burlington to the middle part of the Keokuk limestone, and it occurs in 

 rocks of that age in Iowa, Indiana, Illinois, Missouri, Kentucky and 

 Tennessee. 



